A wireless communications device controls, by using a radio frequency (radio frequency, RF for short) circuit, an antenna to receive and send a radio frequency signal (radio frequency signal), and transmit a radio frequency signal (data transmission), for example, during a voice call or web page browsing. However, the radio frequency signal generates RF radiation, for example, electromagnetic radiation (electromagnetic radiation). RF radiation whose frequency band range is from 3 kilohertz (kHz) to 300 Gigahertz (GHz) damages a human body after the RF radiation is absorbed by the human body. Usually, RF radiation energy that is absorbed by the human body from the wireless communications device is internationally measured by using a specific absorption rate (Specific Absorption Rate, SAR for short). The SAR measures RF radiation energy absorbed by a substance per unit mass in a unit time, and a unit is watt per kilogram (W/kg).
Currently, a specified upper limit value is set for an SAR value of the wireless communications device in some countries, so as to ensure that wireless communications devices introduced to these countries do not cause excessively high RF radiation. For example, both the US Federal Communications Commission (Federal Communications Commission, FCC for short) and the European CE (Conformite Europeenne) specify an SAR standard (SAR compliance criteria) of the wireless communications device. In an SAR standard specified by the US FCC, an SAR upper limit value of the wireless communications device is 1.6 W/kg. That is, RF radiation energy absorbed by each kilogram of human tissue in six minutes is not more than 2 watts (W). In an SAR standard specified by the European CE, an SAR upper limit value of the wireless communications device is 2 W/kg. In two papers entitled “Standard for Safety Levels with Respect to Human Exposure to Radio Frequency Electromagnetic Fields, 3 KHz to 300 GHz” (C95.1-1992) and “Recommended Practice for Measurement of Potentially Hazardous Electromagnetic Fields-RF and Microwave” (C95.3-2002) that are published in the Institute of Electrical and Electronics Engineers (Institute of Electrical and Electronics Engineers, IEEE for short) and the American National Standards Institute (American National Standards Institute, ANSI for short), content of the SAR standard is specifically described.
A method for measuring the SAR value of the wireless communications device is to measure an average RF radiation energy absorption rate at one point of each kilogram of human tissue in a period of time. An SAR calculation formula is:
            S      ⁢                          ⁢      A      ⁢                          ⁢      R        =                  σ        ·                  E          2                    ρ        ,where σ is electric conductivity of an absorber (for example, a phantom) that simulates human tissue, E is a variance (Root Mean Squared, RMS) of electric intensity caused when the wireless communications device generates a radio frequency signal, and p is mass density of the absorber that simulates the human tissue.
A value of electric intensity generated when a radio frequency signal is received or sent by using an antenna is related to transmit power of the antenna. When the transmit power of the antenna is high, electric intensity around the wireless communications device increases, and RF radiation energy correspondingly increases. The SAR value of the wireless communications device may not meet the SAR standard (referred to as “the SAR value of the wireless communications device exceeds a limit” below). FIG. 1a is a schematic diagram of a scenario in which an SAR value of a wireless communications device exceeds a limit. As shown in FIG. 1a, when transmission is performed by using an antenna 1 of the wireless communications device at maximum transmit power, an SAR value generated by the device exceeds a limit. Therefore, to meet a requirement of the foregoing countries for the SAR value of the wireless communications device, the SAR value of the wireless communications device is usually controlled by using a power backoff (Power Backoff) mechanism in the prior art. The power backoff mechanism allows to reduce transmit power of an antenna of the wireless communications device, so as to reduce the SAR value of the wireless communications device.
FIG. 1b is a schematic diagram of reducing an SAR value by using a power backoff mechanism. As shown in FIG. 1b, when transmit power of an antenna decreases, generated RF radiation energy decreases, so that an SAR value of a wireless communications device meets an SAR standard. However, when the transmit power of the antenna decreases, communication performance of the wireless communications device deteriorates. If the wireless communications device is in a weak signal area with relatively poor signal quality, it is likely that a call drop occurs or uplink call quality is poor. FIG. 1c is a schematic diagram of a scenario in which call performance of a wireless communications device deteriorates after antenna transmit power decreases. Therefore, how to ensure communication quality of a wireless communications device while taking account of an SAR standard of the wireless communications device in each country is a difficult subject.